Transforming Waste into Wealth: A New Way to Use Sewage Sludge

Did you know that sewage sludge, the stuff left over after treating wastewater, could be a treasure trove of biogenic carbon? It's true! In the EU alone, over 10 million dry tons are produced each year, containing about 30% carbon by dry weight. The problem? It's wet and full of ash, making it tough to use sustainably. So, scientists set out to find a new way. They cooked up a process called hydrothermal carbonization (HTC) to dry sewage sludge without using lots of energy. They heated it to temperatures between 180 to 270°C for 20 to 120 minutes. This turned the sludge into hydrochar, a solid material. But they didn't stop there. They wanted to make hydrochar useful, so they soaked it in nitric acid (HNO₃) at 70°C for an hour. This leached out most of the ash and some important inorganic compounds, like phosphorus (P) and calcium (Ca), with extraction efficiencies over 95%! The result? A leached hydrochar (LHC) that's more stable and could replace some fossil coal in steelmaking. It also worked well as a reducing agent in blast furnaces, with an average replacement ratio of 40%.